Differential gears are used in principle in all types of vehicles if a differentiation in rotational speed is to be achieved between the driving wheels. This is due to that the wheels on a vehicle rotate at a different velocity when the vehicle turns, due to the fact that the wheels follow tracks with a different radius. However, the differential has the disadvantage that if one driving wheel has lower friction against the ground than the other driving wheel, the wheel with the lower friction will begin to spin and the vehicle loses propulsion power, due to the fact that the differential is torque-equalizing and thus both driving wheels will have the torque which the wheel with the lower friction can transmit.
To remedy this problem, a number of devices have been developed. The simplest is a differential lock, which quite simply locks the drive shafts to one another so that they are forced to rotate at the same velocity. This is normally only used in extreme cases as it places great strain on the transmission and tires.
One development of this is to arrange a differential brake. These comprise a braking device which often consists of disc packs. The brake is activated in various ways and slows down the differences in rotational speed, often due to the braking moment increasing as the driving torque increases. In advanced types of differential brakes, the braking moment can be changed via hydraulics controlled by a computer. This requires the rotational speed differences to be detected by sensors which sense the rotation of the shafts. The systems can be more or less sophisticated. being provided with information regarding the driving torque of the vehicle, steering deflection, velocity, gear selection and the like The differential brakes which are activated by the torque often have a certain preload, i.e. the discs already lie close to one another in the unloaded state. This is done deliberately in this way precisely on account of the fact that a differential gear is torque-equalizing. The preload is required to transmit a certain torque to the driving wheel which has the higher friction as stated above. The preload thus provides a certain straining effect at the differential even when this is not desirable.
The general limitation with differential brakes is that they cannot achieve a certain defined rotational speed difference between the drive shafts, but can only slow down a rotational speed difference which is too great.